Inspectioneering Journal

API Technical Report 939-D, Second Edition, Stress Corrosion Cracking of Carbon Steel in Fuel Grade Ethanol: Review, Experience Survey, Field Monitoring, and Laboratory Testing

This article appears in the May/June 2007 issue of Inspectioneering Journal.
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The American Petroleum Institute (API) is issuing this publication's announcement to inform companies involved in the distribution, transportation, storage, and blending of denatured fuel ethanol of a potential for metal cracking and product leakage from carbon steel equipment in certain portions of the fuel ethanol distribution system. API, with assistance from the Renewable Fuels Association (RFA), has published Technical Report 939-D, Second Edition, that describes cracking events and associated ethanol leaks, the results of related research and field studies, and preliminary guidelines for mitigation and prevention. However, this announcement is being issued at this time so that handlers of fuel ethanol can evaluate the need for additional appropriate precautions.


Fuel ethanol is increasingly being used as a gasoline oxygenate and extender. It is supplied to specifications described in ASTM D 4806 that provides guidance primarily for product purity and functionality of fuel ethanol in gasoline blends used in automotive engines. Once manufactured, fuel ethanol is transported by a number of means (e.g. tanker trucks, rail tanker cars, barges and pipelines) to ethanol distribution facilities and gasoline blending facilities where it is held in storage tanks prior to blending with gasoline. The primary material of construction used in this distribution system is carbon steel. Until recently, fuel ethanol was not widely recognized for its potential to cause cracking of carbon steel. API is issuing this announcement because of the potential impact of cracking and leaks if this problem is not adequately managed.

Field Experience

In the API investigations conducted to date, it is apparent that a number of leaks have occurred in equipment handling fuel ethanol due to cracking of the carbon steel. Approximately two dozen cases of cracking and leaks have been documented in various types of equipment including storage tanks, piping, and associated handling equipment in the distribution system for fuel ethanol, and in an ethanol transport pipelines. However, there have been no reported cases at ethanol manufacturing facilities, nor has cracking been reported after ethanol has been added into conventional gasoline blends with nominally 10 percent ethanol with balance gasoline. Thus far, no cracking events have been reported in carbon steel equipment exposed to E85 fuel blends.

Key Messages

There have been metallurgical investigations conducted in several documented cases and preliminary research has been conducted by API with the assistance of the RFA. Even though the factors that lead to cracking are not completely understood, it is known to occur by a phenomenon referred to as stress corrosion cracking. Based on a survey of companies involved in the distribution and handling of fuel ethanol, several have taken steps to inspect for cracks and mitigate this problem. Inspection techniques have included use of wet fluorescent magnetic particle testing to find cracks on interior surfaces exposed to fuel ethanol. Mitigation techniques used by some companies include thermal stress relief of welds in piping and the use of internal coatings in storage tanks that are resistant to immersion in ethanol, however, these measures are considered interim until more research is conducted. Most recently, laboratory tests have indicated a susceptibility to cracking of carbon steel in E85 blends of fuel ethanol and gasoline however, there have been no documented cases of cracking of carbon steel equipment in E85 in the field.

Additional Information

The following are resources for additional information:


Please contact API at if you have information on failures of carbon steel equipment in fuel ethanol service or in ethanol gas blends.

Printed copies of API TR 939-D, Second Edition, may be purchased for $147.00 each. API members receive a 30% discount on orders. For more information on ordering this and all API publications, visit Second Edition, May 2007 Pages: 172 

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